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RFC 1812 - 4.3.2.5 TOS and Precedence

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### 4.3.2.5 TOS and Precedence

ICMP error messages SHOULD have their TOS bits set to the same value as the TOS bits in the packet that provoked the sending of the ICMP error message, unless setting them to that value would cause the ICMP error message to be immediately discarded because it could not be routed to its destination. Otherwise, ICMP error messages MUST be sent with a normal (i.e., zero) TOS. An ICMP reply message SHOULD have its TOS bits set to the same value as the TOS bits in the ICMP request that provoked the reply.

ICMP Source Quench error messages, if sent at all, MUST have their IP Precedence field set to the same value as the IP Precedence field in the packet that provoked the sending of the ICMP Source Quench message. All other ICMP error messages (Destination Unreachable, Redirect, Time Exceeded, and Parameter Problem) SHOULD have their precedence value set to 6 (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL). The IP Precedence value for these error messages MAY be settable.

An ICMP reply message MUST have its IP Precedence field set to the same value as the IP Precedence field in the ICMP request that provoked the reply.

Next: 4.3.2.6 Source Route

Connected: An Internet Encyclopedia
RFC 1812 - 4.3.2.5 TOS and Precedence